RNA regulation provides a mechanism to rapidly control gene expression in response to stimuli, including environmental changes. This project seeks to generate and utilize structural information to enhance our understanding of these processes with an emphasis on the importance of RNA target specificity for proper gene regulation. In this fiscal year, we have studied the atomic structures and functions of different RNA-binding proteins that regulate germline development and cellular responses to the environment. A major focus of our group is to understand the function of Pumilio/FBF (PUF) proteins. We determined the first crystal structure of a PUF protein in complex with RNA, which allowed us to understand the RNA recognition properties of PUF proteins and have extended these structural studies to a variety of PUF proteins. We have determined crystal structures of engineered proteins with altered RNA recognition specificity and demonstrated that the engineered protein can substitute for the endogenous protein with similar RNA-binding specificity. We have also determined the first crystal structures of a PUF protein and discovered a novel dimeric RNA recognition mechanism. Finally, we have demonstrated a novel mechanism by which a partner protein can act as a rheostat to modulated the RNA-binding affinity of a PUF protein in germline cell development.